The invention relates to a method and an apparatus for determining the yaw angle of a satellite.
In order to effectively control satellites, especially geostationary satellites, the exact orientation of the satellite has to be known. Apart from other values, the roll angle, the pitch angle and the yaw angle, which will described in greater detail further below, have to be measured or estimated. Most geostationary, so called xe2x80x9cthree-axis stabilizedxe2x80x9d satellites are provided with sensors that allow to measure and actively control the roll and pitch angle. In this case, no sensor is provided for measuring the yaw angle. The yaw angle is usually estimated and controlled by means of the roll/yaw coupling that occurs throughout the orbit when an angular momentum bias is present on the spacecraft, for instance when a momentum wheel is spinning inside. Since this coupling is actually very light, a proper yaw angle estimation and correction takes several hours. Although these spacecrafts also are usually equipped with rate measurement assemblies like gyroscopes with a fast measurement response around all three axes, the latter are only used for autonomous attitude measurements over short periods of time for instance when the attitude is expected to be disturbed like during station keeping maneuvers. The reason is that the integrated angle tends to drift away due to the presence of an inherent bias in the measured rates. In addition, when the rate measurement devices are gyroscopes the risk of mechanical wear leads the operators to turn them off whenever their use is not absolutely required.
Even though these types of satellites usually control their yaw angle very acceptably without measuring it, there are cases when a fast yaw measurement is highly desirable like after an unexpected attitude disturbance or when not enough angular momentum bias is present to allow sufficient coupling between the roll and yaw angles, for instance when the momentum wheel has failed. A daily monitoring of the yaw angle profile is also useful to evaluate the health of the attitude control system.
Against this background the technical problem to be solved is to provide a method and an apparatus for determining the yaw angle of a satellite on the basis of sensor measurement signals readily available at the satellite, i.e. without the requirement of additional sensors.
A very advantageous aspect of the invention is the fact that the invention needs no estimation schemes which introduce a considerable delay in computing the yaw angle. Rather the invention makes use of a direct measurement of the yaw angle by means of sensors already present on the satellite. This makes it possible to provide a fast yaw measurement avoiding to collect hours of data before be able to infer a good yaw estimation.
The method is not based on a model matching estimation scheme as mentioned hereabove, but on a direct measurement of the yaw angle purely based on the geometry of the sensors. In other words, this method does not require long data collection periods, but only needs one measurement on two sensors to infer a yaw angle. This is particularly of interest when the spacecraft undergoes some unexpected attitude disturbance. In this case, if a model-matching estimation scheme is used, since the model would not fit the observation, the whole data collection should be restarted after the disturbance for proper yaw estimation. With the method according to the invention, only one measurement, at some point in time, of two offset sensors would be needed before, during or after the disturbance to infer a valid yaw measurement. Furthermore, this method does not suffer any observability problem. Hence, the method is fast and reliable and is perfectly suited for real time or historical monitoring of yaw pointing.
Another important aspect of the invention is to monitor the performances of all different subsystems of a satellite for supervision and error recovery.
This monitoring sometimes highlights deficiencies of some subsystems. Upon deficiency detection, corrective operations are undertaken to either enhance the performance or prevent further damages. Monitoring the proper functioning of the yaw pointing control is part of this overall monitoring task. A poor or unexpected yaw pointing is not only liable to affect the satellite""s mission (poor broadcasting, poor images, . . . ) but is also a clue of poor attitude control, possibly due to on-board hardware or software faults. An usual way to monitor the yaw pointing is to record the roll pointing profile over several hours or days. Indeed, due to the lack of direct yaw sensor mounted on-board of the spacecraft, many of these spacecrafts control or estimate their yaw angle based on the orbital coupling between roll and yaw angles. By recording the roll angle profile over a sufficient amount of time, the ground station can infer the yaw angle profile over the same time span. This is done by applying some numerical estimation scheme to the roll angle data, using a numerical model of the spacecraft""s dynamics and kinematics. For instance, a least square fitting can be worked out in that workframe. These estimation methods are based on the adjustment of the model""s parameters to match the way the roll profile has varied over the period of observation. However, not only these methods require long observation durations for accurate yaw estimation, due to low coupling between roll and yaw, they also require very good modeling of the environmental disturbance torques for proper modeling of the spacecraft dynamics. If, during the roll angle data collection period, anything unexpected happens to the spacecraft attitude that is not taken into account into the numerical model of the spacecraft, like an external disturbance, the resulting estimated yaw angle could be way off the real yaw angle. In this case, the roll angle data collection must be restarted after the unexpected event which further delays the yaw estimation. In addition, depending on the available sensors, the exposed yaw angle estimation method often suffers an observability problem. Indeed, the aforementioned environmental torque modeling is usually achieved through a limited fourier series development of the torques around the yaw and roll axes. If the spacecraft only disposes of a roll angle and a pitch angle measurement device, the constant term of the fourier series of the yaw environmental torque is not distinguishable from the yaw angle itself. Therefore, in this case, the yaw is not estimated as such, but the estimation outputs the sum of the yaw angle and the influence of the constant term of the yaw environmental torque. In this configuration, it is impossible to verify the magnitude of the parasitical effect of the torque""s constant term. If this effect is large, it will significantly bias the whole yaw estimation.
Hence, the method according to the invention can be implemented in real time and allows the operator not only to verify the magnitude of the yaw angle at some point in time, but also to track the profile of the yaw angle throughout hours, days or years. It for instance allows comparisons of daily yaw profiles to make sure there is no attitude control performance degradation or unexpected variation in yaw angle. It could also be looked at if there is any clue of yaw transient (e.g. poor broadcasting).
A further aspect of the present invention deals with the calibration of the yaw angle measurement which is performed as mentioned above.
The theory assumes perfectly stable sensors whose boresights are always pointing in the same direction with respect to the satellite""s coordinate frame. However, real attitude sensors are not perfect. Their reading and pointing are for instance quite sensitive to thermal variation or aging. For instance, due to the daily rotation of a geostationary spacecraft""s body, the structure on which a sensor is mounted follows a daily distortion cycle due to cycling sun exposure. Another source of spurious error can be a initial misalignment of the sensor on the spacecraft body.
Hence, applying the yaw measurement as mentioned above, a further problem occurs in calibrating this yaw measurement also readily available at the satellite without a requirement of additional sensors.
The method according to the invention allows to measure the yaw angle from the reading of two different sensors measuring the roll and/or pitch angles, provided that the reference point of the two sensors are not identical. The description is given basically for geostationary satellites but the invention can be applied directly to satellites which are stationary with respect to any star. The method can also be employed for non circular orbits. The method assumes that the orbit of the spacecraft is known at any time.